1. Field of the Invention
This invention relates generally to the field of Stirling machines connected to a reciprocatable body that is a component of an associated apparatus, the associated apparatus being a load such as a linear alternator driven by a Stirling engine or a prime mover such as a linear motor that drives a Stirling heat pump (cooler), and more particularly relates to an improved link between the piston of the Stirling machine and the reciprocatable component body, for allowing improved optimization of both the Stirling machine and the associated apparatus.
2. Description of the Related Art
Stirling machines have been known for nearly two centuries but in recent decades have been the subject of considerable development because they offer important advantages. Modern versions have been used as engines and heat pumps for many years in a variety of applications. In a Stirling machine, a working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. Each Stirling machine has a pair of pistons, one referred to as a displacer and the other referred to as a power piston and often just as a piston. Some Stirling machines have multiple sets of these pistons. The reciprocating displacer cyclically shuttles a working gas between the compression space and the expansion space which are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter. The shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and/or gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and/or gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in both spaces at any instant of time because the spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the Stirling machine is working as a heat pump or as an engine, as discussed below. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work so it can function as an engine and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm heat sink.
Stirling machines can therefore be designed to use the above principles to provide either: (1) an engine having a piston and displacer driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space and therefore operating as a prime mover driving a mechanical load, or (2) a heat pump having the power piston cyclically driven by a prime mover for pumping heat from the expansion space to the compression space and therefore capable of pumping heat energy from a cooler mass to a warmer mass. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used to generically include both Stirling engines and Stirling heat pumps.
Until about 1965, Stirling machines were constructed as kinematically driven machines meaning that the piston and displacer are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to a mechanical drive linkage. A free-piston Stirling machine is a thermo-mechanical oscillator that is an energy transducer converting energy between thermal and mechanical forms of energy. One of its pistons, the displacer, is driven by working gas pressure variations and pressure differences in spaces or chambers in the machine. The other piston, the power piston, is either driven by a reciprocating prime mover when the Stirling machine is operated in its heat pumping mode or drives a reciprocating mechanical load when the Stirling machine is operated as an engine. Free piston Stirling machines offer numerous advantages including the ability to control their frequency, phase and amplitude, the ability to be hermetically sealed from their surroundings and their lack of a requirement for a mechanical fluid seal between moving parts to prevent the mixing of the working gas and lubricating oil.
Free-piston Stirling machines designed and operated in either the engine mode or the heat pumping mode are capable of being, and have been, connected to a diverse variety of associated apparatuses. Free-piston Stirling engines provide output power in the form of mechanical reciprocation and therefore can be linked as a prime mover to drive mechanical loads as the associated apparatus. These loads include linear electric alternators, compressors, fluid pumps and even Stirling heat pumps. Similarly, free-piston Stirling machines operated in a heat pump mode can be driven as a load by other prime movers as the associated apparatus, including linear motors and Stirling engines.
Stirling machines are often connected to a linear motor or linear alternator. Both an electric linear motor and an electric linear alternator are the same basic device. At times they are referred to collectively as motor/alternator or similar term since both have many identical characteristics. They have a stator, ordinarily having an armature winding, and a reciprocating component body that ordinarily includes magnets, usually permanent magnets, that can reciprocate within the armature winding. The power piston of the Stirling engine is connected to the reciprocating component body of the linear alternator to reciprocate the magnets within the armature winding and thereby generate electric power. Similarly, when a Stirling machine is operated in a heat pumping mode and driven by a linear electric motor, the reciprocating component body of the linear electric motor is connected to the power piston of the Stirling heat pump. Whether the Stirling machine is operated as an engine or a heat pump, the power piston of the Stirling machine is, in the prior art, directly connected to the reciprocating component body of the linear motor or alternator by a rigid or fixed connection or link. Consequently, the piston of the Stirling machine and the reciprocating component body of the linear alternator or linear motor reciprocate as a unit at the same frequency and the same amplitude of oscillation. This direct connection is typically accomplished by mounting the magnets to a magnet carrier or framework that is mounted to the power piston, but sometimes they are connected by a connecting rod. Other combinations of a free-piston Stirling machine and an associated apparatus also have the power piston of the Stirling machine linked by a rigid connection to the reciprocating body of the associated apparatus so that they reciprocate as a unit.
Although the prior art discloses a large quantity of combinations of a free-piston Stirling machine and an associated apparatus, FIGS. 1 and 2 illustrate a representative example of a free-piston machine coupled to a electric linear motor or linear alternator as the associated apparatus. The Stirling machine and the linear motor/alternator are often mechanically integrated to some extent so they do not appear in FIG. 1 as two easily distinguished machines in a simple side by side arrangement. Referring to FIG. 1, a linear electric motor/alternator 10 has an armature winding 16. A Stirling machine 12 has a power piston 18 that reciprocates axially within a cylinder 19 at an operating amplitude and frequency of reciprocation. A reciprocating component body of the motor/alternator comprises a magnet carrier 17 that is rigidly fixed to the power piston 18 and a series of permanent magnets 20 that are fixed to and supported by the carrier 17. The permanent magnets 20 reciprocate axially (parallel to axis 21) in an air gap within the armature winding 16 at the operating frequency of reciprocation. Consequently, because the piston 18, the magnets 20 and their support carrier 17 are integrated together, the piston and the reciprocating body of the motor/alternator are a single unit with power piston 18 and the magnets 20 rigidly connected together and therefore reciprocating at the same amplitude and frequency. The displacer 22 of the Stirling machine is fixed to one end of a connecting rod 24 and the opposite end of the connecting rod 24 is connected to a planar spring 25 so that the displacer 22 and its connecting rod 24 can also reciprocate axially at the operating frequency of reciprocation. The Stirling machine also has heat exchangers 26 and 28 and an interposed regenerator 30 through which working gas is shuttled between the expansion space A and compression space B.
The operating frequency of a combination like that shown in FIG. 1 is typically approximately the resonant frequency of the mass of the piston 18 and its attached masses and the spring forces, principally the spring forces of the planar spring 25 and the gas spring forces of the working gas within the hermetically sealed machine. Free piston Stirling machines typically operate in the frequency range from about 30 Hz to 120 Hz. The operating frequency of a Stirling machine may vary slightly under differing operating conditions, but ordinarily that variation is very small, not exceeding a few Hz. A Stirling machine may, for some applications, be operated at a frequency that is near but slightly displaced from its natural frequency of oscillation, but is operated at a frequency within the range of its resonance peak. However, the amplitude of the power piston 18, and with it the amplitude of the reciprocating body of the motor/alternator 10, may vary considerably as a function of variations in operating conditions, such as the electrical power output of a linear alternator.
FIG. 2 is a more diagrammatic illustration of the combination of a Stirling machine and a linear motor/alternator that is illustrated in FIG. 1. FIG. 2 is more simplified for facilitating explanation of the invention and uses the same reference numerals used in FIG. 1 for identifying the same parts. The rigid connection of the power piston 18 of the Stirling machine to the magnet carrier 17 and its magnets 20, which form the reciprocating component body of the motor or alternator, is illustrated in FIG. 2 as bars or connecting rods 34 rigidly connecting the magnet carrier 17 to the power piston 18.
Whenever a free-piston Stirling machine is connected to an associated apparatus that is either a load that it drives or a prime mover that drives it, the combination involves a connection and interaction of two dynamic systems. An engineer designing such a combination typically attempts to optimize one or more characteristics of the combination by finding an optimum operating point for the combined system. One characteristic that is important to optimization is the amplitude of oscillation. Unfortunately, because the dynamic systems are so different, it is not unusual for the optimum operating point for each system to be different from the optimum operating point for the other system. Since the optimum operating points of the two systems do not coincide, the traditional approach is to make the best available engineering compromises and tradeoffs between the two systems.
For example, the design of a high power electrical generating system, in which a free-piston Stirling engine drives a linear alternator, involves the interaction of the dynamics of the thermodynamic cycle of the engine and the dynamics of the electromagnetic alternator system. Optimum linear power densities occur at higher amplitudes of alternator oscillation. However, modifying the design of free-piston Stirling engine so that it provides a greater amplitude of oscillation that is closer to the optimum alternator operating amplitude, eventually leads to a free-piston Stirling engine that can not reciprocate the alternator effectively. In other words, the operating amplitude for optimum alternator operation does not coincide with the operating amplitude for optimum free-piston Stirling engine operation.
The necessity for engineering compromises and tradeoffs resulting from the lack of coincidence of the optimum operating amplitude of reciprocation for each of two interconnected but very different dynamic systems also applies to other combinations in which a free-piston Stirling machine is connected to an associated apparatus. The traditional direct, rigid connection of the piston of the free-piston Stirling machine to its load or prime mover limits the engineer to combined systems in which both have the same operating amplitude of reciprocation.
It is therefore a purpose and feature of the present invention to provide an improvement in a free-piston Stirling machine connected to an associated apparatus that is a load or prime mover, wherein the improvement permits the free-piston Stirling machine and the associated apparatus to operate at different amplitudes of oscillation and thereby allow better optimization of each.